An internal combustion engine has a number of cylinders, each connected to an intake manifold via at least one intake valve, and to an exhaust manifold via at least one exhaust valve. The intake manifold receives fresh air (i.e. outside air) along a feed conduit regulated by a throttle valve, and is connected to the cylinders by respective intake conduits, each regulated by at least one intake valve.
A swirl system has recently been proposed to vary the cross section of the intake conduits when the engine is running and as a function of engine speed (i.e. drive shaft rotation speed). At low engine speed, the air flow section of the intake conduits is reduced to produce swirl in the air intake to improve the air-fuel mixture in the cylinders. By swirl-enhancing the mixture, all the injected fuel is burnt, thus reducing combustion-generated pollutant emissions. At high engine speed, the air flow section of the intake conduits is maximized to completely fill the cylinders and so generate the maximum possible power.
To vary the air flow section of the intake conduits, each intake conduit has two parallel channels, only one of which can be completely closed by a throttle valve. At low engine speed, the throttle valves are closed to reduce the air flow section of the intake conduits; and, at high engine speed, the throttle valves are opened to maximize the air flow section of the intake conduits.
Various design solutions for swirl system intake manifolds of the type described have been proposed, but comprise a large number of component parts and are complicated to assemble.